Can I join the dark side AND get the crossbow? I'm currently at the fort of dawnguard and I dunno if talking to the leader will kill my chances of joining the other side.

There is no switching sides. You can still get your hands on a basic crossbow but you won't be able to get any upgrades or better versions as a vamp. You can still temper it, but that'll be your lot.

You have to talk to Isran first regradless of which side you want to join.

If you want to upgrade the crossbow, and complete all the quests that are crossbow related, then becoming a Dawnguard is your only option. Once you choose your alignment, you can not change to the dark side "Luke"